The invention relates to a method for discharging an intermediate circuit capacitor of an intermediate voltage circuit converter, the electronic power converter on the line side of which has power semiconductors that can be disconnected and which is able to be connected to an AC supply network by means of a series circuit able to be bypassed by means of a line contactor comprising a charging contactor and a pre-charging resistor, a transformer and a main switch.
With intermediate voltage circuit converters a pre-charging circuit is generally present. The intermediate circuit capacitor is charged with this pre-charging circuit. The charge current is set by selecting the pre-charging resistor. The charge current may not exceed a maximum diode current of the inverse diode of the switchable power semiconductor of the power converter on the line side. If no pre-charging resistor were present a charging current would be set which would exceed the maximum current carrying capability of the line-side diodes. This would result in the destruction of these diodes. As soon as for example 80% of the maximum charge state of the intermediate circuit capacitor is reached, the pre-charging resistor is bypassed by means of a line contactor. This connects the line-side converter by means of a mains transformer to an AC power supply network.
For maintenance purposes or to rectify errors it must be ensured that the intermediate circuit capacitor is discharged within a relatively short time after the intermediate voltage circuit converter has been switched off. At least the voltage dropping at the intermediate circuit capacitor should not be greater than 60V DC. This means that dangerous currents cannot flow through the bodies of maintenance personnel coming into contact with this DC voltage.
In commercially available voltage intermediate voltage circuit converters the intermediate circuit capacitor is discharged by means of a switchable resistor. This resistor is switched electrically in parallel to the intermediate circuit capacitor. Additional components such as a contactor or a switchable power semiconductor and a resistor are thus needed for the discharging.
A method is known from DE 10 2007 047 713 A1 for discharging a high-voltage network which uses the available pre-charging resistor for this purpose. The circuit, which has a DC voltage capacitor, is a high-voltage network present in a hybrid vehicle. In the simplest case this high-voltage network consists of a battery, a power converter with DC voltage capacitor, one or more electrical machines, further high-voltage loads and a cable loom, which connects all high-voltage components to each other electrically-conductively. The high-voltage battery is able to be connected to a positive pole of the DC voltage capacitor by means of a bridgeable series circuit comprising a pre-charging resistor and a switch. The negative pole of the DC voltage capacitor is also able to be connected by means of a further switch directly to a negative pole of the high-voltage battery. For the discharging process a discharging contactor is used, which likewise has a simple closing contact or switch. The discharging contactor is connected to the pre-charging resistor such that, with closed contacts of the discharging contactor, the pre-charging resistor is connected electrically in parallel to the DC voltage capacitor.
Through this measure the known discharging circuit is restricted to a contactor, since the pre-charging resistor is not just used for charging purposes but also for discharging purposes. Through the dimensioning of the pre-charging resistor a discharging of the DC voltage capacitor is achieved far more quickly than with a separate discharging circuit. Although the outlay of the discharging circuit has been very much reduced, an additional component, namely a contactor, is still needed, which demands corresponding space and must be connected to the circuit which features a DC voltage capacitor.
The object of the invention is now to specify a method with which an intermediate circuit capacitor of a voltage intermediate circuit converter described above can be discharged.